(Not Applicable)
1. Technical Field
The invention concerns wireless communications equipment, and more particularly improvements to wireless base stations for cellular communications.
2. Description of the Related Art
Conventional wireless cellular communications systems have a common architecture in which one or more defined cell sites are formed by the placement of one or more base transceiver stations within a geographic area. A cell site is typically depicted as a hexagonal area in which a transceiver is located such that a radio communication link can be established between the cellular system and a plurality of mobile stations within the defined geographic area for the cell. A variety of standards exist for cellular telephone communications. For example, a common cellular system in the United States makes use of the advanced mobile phone service (AMPS). Other common systems include Nordic mobile telephone service (NMT), total access communications service (TACS), global system for mobile communications (GSM), IS-136 TDMA, and IS-95 CDMA systems.
Each of the above-identified systems makes use of a standard architecture associated with the particular system. For example, the basic architecture of a GSM type network is comprised of a base station subsystem (BSS) which includes a base station controller (BSC) and several base transceiver stations (BTS), each of which provides at least one radio cell with one or more radio frequency (RF) channels for communications with mobile subscribers. The purpose of the BSC is to control each of the BTS units within a region. This control process involves several functions, including allocating and selecting RF channels for transmitting each call and controlling handovers of calls from one BTS to another within the BSC""s control region. When a mobile subscriber seeks to place a call, the mobile station will attempt to contact a local BTS. Once the mobile station establishes contact with the BTS, the mobile unit and the BTS will be time synchronized for permitting time division multiple access (TDMA) communications. Subsequently, a dedicated bidirectional signaling channel will be assigned to the mobile subscriber by the BSC.
Finally, the BSC will also set up a switching route to connect the mobile subscriber to a mobile switching center (MSC). The MSC provides a communication link from the BSC, and, as a result, all of the BTSs controlled by the BSC, to the public switched telephone network (PSTN), and performs all necessary call and signal routing to other networks to support mobile communications. This arrangement permits a mobile user or subscriber to move from cell to cell and still maintain service. This architecture is also particularly advantageous as it makes possible reuse of carrier frequencies from one cell to another.
The GSM system is designed to work in the 900 MHz and 1800 MHz bands, as well as the 1900 MHz PCS band in North America. GSM is essentially an all digital service. Each RF channel in the GSM system provides eight digital time slots due to the use of TDMA technology. Each of the RF channels are spaced 200 kHz apart from adjacent channels. The eight time slots supported by the TDMA technology enables each RF channel to be shared by more than one user. With TDMA, each user""s voice communication is converted to a digital signal, which is allocated among one of the time slots in an assigned RF channel before being transmitted. In a GSM system, TDMA requires that all user subscriber signals using a single RF channel must arrive at the BTS at the proper time. Overlap of signals from the various mobile stations must be avoided and are ensured by proper signal transmission timing.
BTS equipment used in conventional cellular communication systems typically designates specific RF and signal processing equipment for each individual RF channel allocated to the BTS. This designation can most likely be attributed to the fact that each BTS has been conventionally configured to provide communication capability for only a limited number of predetermined channels in the overall frequency spectrum that is available to the service provider. In any case, each BTS is conventionally assigned at its initialization or during its construction, a set of RF channels on which it can communicate with subscribers. These assigned RF channels are generally carefully chosen so that the potential for interference between cells is minimized.
Within a particular BTS, a single omni-directional antenna can be used to receive and transmit signals to all mobile subscribers. However, a more common approach makes use of a plurality of directional antennas at the BTS site to split a cell into separate sectors, effectively transforming the one cell into multiple cells. Dedicated hardware in the BTS units are typically provided for handling communications for each sector. When using a sectorized approach, the RF channels assigned to a particular BTS must be further allocated among each of the sectors, since interference can be caused if multiple sectors processed by the BTS are operated on the same frequency. Each BTS is provided with DSP units to support communications processed by the particular BTS to which the DSP unit has been assigned. Conventional DSP units in such systems are pre-configured to operate on only the particular RF channels which have been assigned to a specific sector of the BTS.
Thus, DSP units are not generally fungible as between sectors of a particular BTS, and therefore these DSP units cannot be allocated from one sector to another. In cell sites that experience heavy traffic, this limitation can result in a poor allocation of system resources.
In particular, one of the problems with using sectorization in wireless base stations concerns trunking efficiency. Normally, a fixed number of RF carriers is assigned to a sector with the BTS concentrating traffic through a common interface to the PSTN. In many instances, traffic needs in one sector can occasionally exceed the sector""s RF and processing resources while resources are available in another sector. However, because the number of RF channels allocated to a sector is fixed in conventional BTS system, those resources are blocked and left unused, lowering the trunking efficiency of the base station.
Omni-directional BTSs, i.e., those that are not sectorized, do not suffer from blocking. For example the Erlang capacity of a 2-carrier omni-directional base station is approximately the same as a sectorized base station using 3 carriers (one per sector). In this regard, it is well known that the sectorized system requires more resources. However, omni-directional base stations do not provide as high a degree of coverage as sectorized systems due to lower antenna gain. Another problem with omni-directional systems is that they cannot take advantage of higher frequency reuse schemes, therefore lowering overall system capacity.
Some companies, such as AirNet Communications Corporation of Melbourne, Fla., use a broadband base station (BBS) rather than the BTS described above. Such systems are disclosed in U.S. Pat. Nos. 5,535,240 and 5,940,384. In this BBS, a broadband transceiver is used for transmitting and receiving a single composite wideband RF waveform that is comprised of a number of frequency channels, rather than the multiple narrow-band transceivers used in the BTS for transmitting and receiving individual frequency channels. By replacing the narrow band transceivers of the BTS with a broadband transceiver, this architecture reduces the number of transceivers required to process a given number of frequency channels; however, this alone still does not solve the trunking problem associated with the BTSs. The architecture and configuration of conventional BBSs may still suffer from limited trunking efficiency, as the BBS can still only process a fixed number of calls due to dedicated processing resources serving a specific transceiver and therefore a specific sector.
In a BBS system, one transceiver supports multiple carriers per sector, i.e., each RF channel is not assigned its own unique transceiver and processing hardware. The number of RF channels supported in each sector is therefore not constrained by RF transceiver equipment. Accordingly, RF carriers can be dynamically re-configured to meet changing traffic patterns in the multiple sectors of the BBS. For example, sectors facing major roadways can be more heavily used during morning and evening hours, while sectors directed toward population areas are more used during the middle of the day. Dynamic RF carrier allocation coupled with a broadband transceiver system in a sectorized BBS overcomes the blocking and inefficiency problem of a conventional sectorized BBS that occur when traffic needs in one sector of the BBS exceed its RF and processing resources while resources in another sector of the BBS are left unused. In addition to overcoming the blocking problem, the dynamic allocation of RF carriers of the invention retains the advantages of higher coverage and higher frequency reuse which are normally associated with cell sectorization.
These and other objects of the present invention are achieved by the subject method for dynamically allocating signal processing resources in a wireless multichannel broadband base station (BBS) for a cellular communications network. The method includes the steps of determining a number of available channel processor (CP) resources which are unused among a set of RF processing chain (RFP) resources allocated to a communications cell. Each RFP processes a single frequency channel and contains a plurality of CPs. The plurality of CPs process at least one of a plurality of traffic channels contained on said frequency channel. In response to notification of a subscriber call to be processed by the BBS, a determination is made as to whether the number of available CP resources is at least one. If so, the call is assigned to the available CP resource.
The method can also include the steps of allocating an available RFP resource to the cell if the number of available CP resources in the cell is less than one; and incrementing the number of available CP resources in the cell by a number of CP resources supported by said RFP. In addition, the method can further include the step of decrementing a number of available RFP resources in the BBS after the step of allocating one of the CP resources to process a call. The method can also include the step of decrementing the number of available CP resources by one after assigning a call to an available resource.
Subsequently, the subject method can additionally include the step of rejecting the call if all RFP resources of the BBS are in use, and all CP resources of those RFP resources already allocated to the cell in which the call is initiated are also in use. When a call is rejected, the subject method can include the step of incrementing a count of rejected calls each time a call is rejected for lack of sufficient resources.
In the subject method, the number of CP resources in the cell is determined by counting the total number of CP resources assigned to the cell, and decrementing that total number by the total number of active subscriber calls in the cell and the number of CP resources assigned for handling control channel traffic in the cell. The method can also include incrementing the number of available CP resources in the cell when the call is terminated.
Furthermore, the method can include the step of deallocating the RFP resource from the cell when termination of the call results in all CP resources of the RFP resource becoming available. Upon deallocating the RFP resource from the cell, the subject method can also include the step of decrementing the number of available CP resources of the cell by a number of CP resources supported by the RFP when the RFP is deallocated.
With respect to call handover in the BBS, the subject method can further include handing over the call to a target cell of the same BBS; and assigning the call to an available CP resource of the target cell. This method includes reallocating the RFP resource from the cell to the target cell if handover of the call from the cell results in all CP resources of the RFP resource allocated to the cell becoming available. The method includes, prior to the reallocating step, determining whether all other RFP resources of the BBS are in use, and whether all CP resources of the other RFP resources already allocated to the target cell are also in use.
In dynamically allocating RF carriers within a BBS, the present method can also include periodically consolidating a set of active subscriber calls to a minimum number of RFP resources assigned to the cell.
An additional embodiment of the invention is a wireless multichannel broadband base station (BBS) for a cellular communications network including a programmable central processor (CPU) for determining a number of available channel processor (CP) resources which are unused among a set of RF processing chain (RFP) resources allocated to a communications cell. The CPU is responsive to notification of a subscriber call to be processed by the BBS. In response to the notification of the subscriber call, the CPU determines if there is at least one available CP resource in the BBS, and assigns the call to one of the available CP resources. The CPU can also allocate an available one of the RFP resources to the cell if the number of available CP resources is less than one, and can increment the number of available CP resources by a number of CP resources supported by the RFP.
According to another aspect of the invention, the CPU can deallocate the RFP resource from the cell when termination of the call results in all CP resources of the RFP resource becoming available. Furthermore, the CPU can hand over a call being processed within the BBS to a target cell of the same BBS; and can assign the call to an available CP resource of the target cell. In addition, when such a handover is performed, the CPU can reallocate the RFP resource of the BBS from the cell to the target cell if handover of the call from the cell results in all CP resources of the RFP resource allocated to the cell becoming available. Also, the CPU can include the ability to periodically consolidate a set of active subscriber calls to a minimum number of RFP resources assigned to the cell.